1. Field of the Invention
The present invention relates to a composite electrode and a lithium-based (Li-based) battery using the same, and especially relates to a composite electrode coated with microstructured diamond films and a Li-based battery using the same.
2. Description of Related Art
Currently natural graphite (NG) is used for making electrodes for Li related batteries (LB) because of its low cost (a target price below US$10/Kg), high Coulombic efficiency, low volume expansion after whole lithiation (˜10%), and high electronic conductivity (0.4−2.5×104 S cm−1 on the basal plane). However, LB using NG electrodes has the problem of large irreversible capacity loss and short cycling life due partly to the reactivity of natural graphite with organic electrolytes. Reactions between natural graphite and the electrolyte may form solid-electrolyte-interphase (SEI) films and the generation of organic gases such as hydrogen, CO, CO2, methane, ethylene, and propylene within interlayers of NG crystals, which may cause irreversible capacity loss and damages to the natural graphite structure, and reduce cycling performance of the battery.
Excessive chemical and electrochemical reactions between NG and the electrolyte and the subsequent deposition and accumulation of undesirable electrically insulating SEI on the surface of the NG electrodes worsens at the electrode surface area where high local charging and discharging current density (hot spots) occurs.
Dendritic growth of SEI compound occurs in graphite electrode areas of high reactivity and high charging or discharging current density, leading to the electrical shorting and hazardous failure of the LB. This poses a major huddle against practical applications to electrical vehicles, for which natural graphite based economic LB exhibiting high-rate charging and discharging cycle performance are required.
Attempts have been made to suppressing the undesirable electrolyte-graphite reactions have shown limited success. Most of the methods involve coating NG with polymer, alkali carbonate, and lithium benzoate. These coatings have shown some success to a limited extent depending on the electrochemical characteristics of the specific coatings.
Nanocrystalline diamond (NCD) films, with grain size in the 10-999 nm range, and particularly ultrananocrystalline diamond (UNCD) films (2-10 nm grain size) are known. They are formed mainly with sp3 bonded carbon atoms, and exhibit high hardness, exceptional tribological properties, chemical inertness, biocompatibility, negative electron affinity with proper chemical treatment. These films have been used in, for example, coating tools to reduce wear and tear.
It is also known that nitrogen atoms, when incorporated in the grain boundaries of UNCD and NCD films, satisfy dangling carbon bonds and provide electrons for conduction through the grain boundaries. The UNCD films with nitrogen in the grain boundaries are named N-UNCD. The NCD films with nitrogen in the grain boundaries are named N-NCD. Boron atoms, when substitute carbon atoms in the lattice of the diamond grains provide holes to the valence band via a true p-type semiconductor doping mechanism. The UNCD films with boron atoms substituting C atoms in the diamond lattice are named B-UNCD. The NCD films with boron atoms substituting C atoms in the diamond lattice are named B-NCD.
N-UNCD and N-NCD are known to possess a high density of grain boundaries consisting of substantial sp2 carbon bonds, including graphitic carbon phases, and exhibit excellent chemical and electrochemical inertness, high mechanical strength, and good electrical conductivity with nitrogen atoms being incorporated into the grain boundaries.